Add mergeCells() function
Add ZipReader and XmlStreamReader
Update .gitignore
Try add a new class QXlsx::Document
Add read xml function for Relationships
Using QXlsx::Document instead of QXlsx::Workbook
Able to read the document properties back now!
Update the readme file
Make constructor of Workbook private
Add read xml function for SharedStrings
Add convenient function saveToXmlData() for worksheet